Theory for Hydration Forces in Thin Films of Aqueous Electrolytes

A theory is presented to quantify the electrostatic forces in thin aqueous electrolyte films between two charge regulating surfaces. The Poisson-Boltzmann equation (PBE) is modified to include the effects of dielectric saturation of the double layer and the corresponding change in hydration free energy of ions. The results obtained agree remarkably well with the experimental data of Pashley (1981) and Israelachvili et. al. (1978) at low electrolyte concentration and pH. At higher concentration and pH, the model provides qualitative agreement with experimental observations. Finite ion sizes needs to be incorporated into the PBE to obtain quantitative agreement with experimental data in the high ion concentration range. The reduction in dielectric constant with increasing electric field increases the hydration energy of the ions in the double layer, giving rise to a repulsive hydration force. It is concluded that dielectric saturation of the double layer near a charged interface plays an important role in the electrostatic interactions in thin electrolyte films.Petroleum and Geosystems Engineerin